In principle, lithium batteries offer the unique advantage of reaching unsurpassed volume and specific energies that render them particularly interesting for a wide range of applications, whether stationary or mobile, from microelectronics, portable electronics to large installations for electrical vehicles or hybrids.
These systems generally use liquid aprotic electrolytes mainly of the lithium-ion type, and more recently polymer electrolytes, the latter being either of the dry solvating type polymer, working between 40 and 100° C. with lithium anodes, or of the gelified type, using a solvating or non-solvating polymer, and working at room temperature because of the addition of aprotic polar liquid solvents associated with lithium-ion type electrodes having cathodes working at elevated tensions (˜4V). The extreme operating conditions of temperature or tension of these systems cause the various components of the generator to age under cycling and/or in function of the time.
The components aging is evident at the current collectors level, and results in the formation of passivation films or in the degradation of the surface of the collectors because of the reactions between the components of the generator, namely the active materials of the electrodes, and the chemical components of the organic electrolyte. The formation of such films, more or less insulating, at the interfaces, significantly alters the quality of the electronic exchanges between the collectors and the electrode active materials, which are generally present in the form of composite.
In a polymer electrolyte medium, the effect of the passivation phenomenons is sometimes amplified because in the solid state, the products formed by the reactions of the organic solvent, the lithium salt, the electrode materials or from other components of the generator, tend to accumulate at the interface because of the lack of the convection of the solvent or the solubilization of the films formed, or because of the lack of corrosion/dissolution reaction of the metal and the renewal of the exchange surface. The attack of the collectors or the formation of passivation films at the surface by the oxidation-dissolution of the metallic conductor is generally caused by electrochemical reactions, namely oxidation or reduction, initiated by radicals, acid-base reactions or oxidation-reduction chemical reactions more or less catalyzed by the materials present. FIGS. 1a) and 1b) illustrate a collector/electrode composite assembly and the localization of the passivation film at the interface collector/electrode after cycling.
Passivation phenomenons are particularly evident in the case of aluminium collectors, which are frequently used because of their low cost and their thermal and electrical conduction properties with cathode associated with end of charge voltages frequently higher than 3 and even 4 volts.
In lithium-ion type systems using liquid electrolytes or gelified polymers with liquids, the corrosion of the aluminium of the cathode collector is generally prevented through the use of a salt or a fluorinated additive of the type LiBF4 and LiPF6 that easily form a fluorinated film at the surface of the aluminium, or with an oxidative anion ClO4−, thus preventing deep corrosion or dissolution of the aluminium collector. With other particularly stable fluorinated salts such as TFSI of formula (CF3SO2)2NLi, the corrosion of aluminium above 4 volts can lead to the complete disintegration of the collector.
In dry polymer medium, the formation of passivation films on the aluminium collector of a vanadium oxide-based cathode (V2O5) does not lead to the dissolution of the collector, but rather to the formation of passivation films more or less insulating, that increase the electrical resistance between the collector and the composite cathode. There is then observed the formation of oxygen and fluorine-based oxidation films of aluminium, which are visible under electronic microscopy, that reach thicknesses higher than that of alumina films initially present at the surface of the aluminium. Such films are more or less electrical insulators and thus impair the passage of electrons between the collector and the electronic conduction and active materials present in the cathode.
It has been known for a long time to protect the metallic current collectors of electrochemical accumulators from passivation/dissolution phenomenon by coating the latter with an electronically conductive carbonated coating that is not very oxidizable. Generally, carbon black dispersions in organic or mineral binders are used in the form of a layer more or less impermeable to the electrolyte of the generator to prevent electrochemical corrosion phenomenons. Further, these coatings prevent a direct contact of the collector with the electrode active materials (see for example U.S. Pat. No. 5,262,254). Such solutions are used successfully in various commercial applications. However, none of them is perfectly satisfactory, particularly when the electrochemical generators are used in extreme conditions as described above, and over extended period of times, notably because of the lack of impermeability and chemical or electrochemical stability of the organic binders, or the metallic conduction additives or conjugated polymers.
U.S. Pat. No. 5,580,686 (Fauteux et al.) describes a carbon-based protective coating (“primer”) dispersed in a metallic polysilicate used in an electrolytic cell of the lithium-ion type containing a cobalt oxide cathode and a graphite anode. The polysilicates comprise several limitations because of their strong basicity. For example, they are reactive towards acidic electrode active materials such as vanadium oxide. Further, they are chemically reactive with iron phosphate-type materials. The basic character renders them incompatible with conduction additives made of conjugated polymers of the polyaniline type, doped polypyrole type etc.
In most applications, carbon is generally the preferred additive because of its high chemical inertia and its resistance to electrochemical corrosion.